Generally attached to another term--like gender-fluid or fluid-sexuality. It describes an identity that is on a continuum of sexual orientation and/or gender identity.
Recap: Biological Sex, Gender Identity, Gender Expression, Sexual Orientation
The possibilities are endless...
Biological Sex - your parts - female = vagina, ovaries, XX chromosomes; male = penis, testes, XY chromosomes; intersex = a combination of the two (either internally, externally or both)
Gender Identity - what you think and feel you are regardless of your anatomy and chromosomes